This invention relates to traveling wave tubes, and, more particularly, to a termination output coupler between an output end of a traveling wave tube and an output device such as a waveguide.
Traveling wave tubes are used to amplify signals in microwave systems. For example, traveling wave tubes may be provided in satellite communications systems to amplify the signals received from earth before their retransmission back to earth.
The traveling wave tube includes an input coupling element, an output coupling element, and a barrel therebetween. The barrel is typically made of a thermally and electrically conductive metal such as annealed copper, although other materials may be used. A metallic helix or other type of slow-wave propagation structure extends through the interior of the barrel and transmits a microwave signal. The metallic slow-wave propagation structure is supported by dielectric rods from the inner wall of the bore of the barrel. The dielectric rods serve to position the metallic slow-wave propagation structure, and also to conduct heat from the metallic slow-wave propagation structure to the barrel, where the heat is dissipated. A properly controlled electron current flowing through the interior passage of the slow-wave propagation structure transfers energy to the microwave signal flowing in the slow-wave propagation structure, thereby amplifying the microwave signal.
In one common application, the output of the traveling wave tube is coupled to an output waveguide. The coupling includes a slow-wave propagation structure sleeve which attaches to the adjacent end of the metallic slow-wave propagation structure, and a second sleeve having a slip fit to the output waveguide. The outer surface of the slow-wave propagation structure sleeve and the inner surface of the second sleeve are slip fitted to each other. By adjusting the exact position of the sleeves, an adequate radio frequency match is obtained between the slow-wave propagation structure and the output waveguide. This coupling approach is operable and is widely used.
The inventors have recognized that the conventional coupling using the slow-wave propagation sleeve structure, while operable, has some drawbacks. There is electrical loss at the two slip joints. Each of the two joints offers thermal resistance to the heat which must be removed by radial outward diffusion to maintain the materials within their safe operating temperature limits. The sleeve-within-a-sleeve configuration limits the interior space available for the electron beam, and increases the likelihood of undesirable electron beam interception before the beam can be collected. This structure is also sensitive to environmental effects such as temperature changes and mechanical forces such as vibration.
There is therefore a need for an improved design to the traveling wave tube system, which improves its efficiency and operation while still allowing an adequate radio frequency match to be realized. The present invention fulfills this need, and further provides related advantages.
The present invention provides a traveling wave tube system having an output coupler to an output waveguide. The coupling has allow electrical and thermal loss. It also allows diametral expansion of the electron beam of the traveling wave tube after it leaves the interception region. The thermal and electrical efficiencies of the traveling wave tube system are thereby improved, and the system is capable of handling greater power, as compared with prior coupling approaches. The present coupling is less sensitive to environmental influences, and is more readily fabricated and assembled.
In accordance with the invention, a traveling wave tube system comprises a traveling wave tube, including a hollow barrel, an elongated, hollow slow-wave propagation structure affixed within the barrel and having an interior passage, an electron beam source operable to produce an electron beam within the interior passage of the hollow slow-wave propagation structure, and an input coupler at a first end of the slow-wave propagation structure. The slow-wave propagation structure is preferably a metallic helix. An output waveguide, typically rectangular in cross section, is disposed at a second end of the slow-wave propagation structure. There is an output coupler between the second end of the slow-wave propagation structure and the waveguide. The output coupler comprises a single integral hollow termination body having an inner surface and an outer surface. The slow-wave propagation structure contacts the inner surface of the termination such that the electron beam produced by the electron beam source passes through an interior of the single integral hollow termination body, and the waveguide contacts the outer surface of the single integral hollow termination body, preferably in an interference fit. One or both of the facing surfaces may be coated with gold to improve the electrical and mechanical contact at the facing surfaces. Ordinarily, a set of periodic magnet pole pieces is positioned adjacent to an external surface of the barrel, or some other technique is provided to confine the electron beam.
In the preferred structure, the waveguide includes a stop surface, and the outer surface of the termination body includes a shoulder sized to engage the stop surface. This stop precisely positions the slow-wave propagation structure relative to the waveguide. Desirably, the inner surface of the hollow termination body is substantially circular in cross section, and the diameter of the cross section of the inner surface of the hollow termination body increases with increasing distance from the slow-wave propagation structure. This allows the electron beam to expand radially after it has exited the slow-wave propagation structure.
The present output coupler design requires only a single interface, rather than the two interfaces of the prior art approach, and that single interface has an interference fit rather than a slip fit. These changes reduce the thermal and electrical impedances associated with the coupling, resulting in improved thermal and electrical performance of the system. They also eliminate the possibility of leakage of electromagnetic energy through the slip-fit joints. The traveling wave tube system is therefore able to carry greater power and operate more efficiently. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.